Positive ignition engines cause combustion of a hydrocarbon and air mixture using spark ignition. Contrastingly, compression ignition engines cause combustion of a hydrocarbon by injecting the hydrocarbon into compressed air and can be fuelled by diesel fuel, biodiesel fuel, blends of diesel and biodiesel fuels and compressed natural gas. Positive ignition engines can be fuelled by gasoline fuel, gasoline fuel blended with oxygenates including methanol and/or ethanol, liquid petroleum gas or compressed natural gas.
Ambient PM is divided by most authors into the following categories based on their aerodynamic diameter (the aerodynamic diameter is defined as the diameter of a 1 g/cm3 density sphere of the same settling velocity in air as the measured particle):                (i) PM-10—particles of an aerodynamic diameter of less than 10 μm;        (ii) Fine particles of diameters below 2.5 μm (PM-2.5);        (iii) Ultrafine particles of diameters below 0.1 μm (or 100 nm); and        (iv) Nanoparticles, characterised by diameters of less than 50 nm.        
Since the mid-1990's, particle size distributions of particulates exhausted from internal combustion engines have received increasing attention due to possible adverse health effects of fine and ultrafine particles. Concentrations of PM-10 particulates in ambient air are regulated by law in the USA. A new, additional ambient air quality standard for PM-2.5 was introduced in the USA in 1997 as a result of health studies that indicated a strong correlation between human mortality and the concentration of fine particles below 2.5 μm.
Interest has now shifted towards nanoparticles generated by diesel and gasoline engines because they are understood to penetrate more deeply into human lungs than particulates of greater size and consequently they are believed to be more harmful than larger particles, extrapolated from the findings of studies into particulates in the 2.5-10.0 μm range.
Size distributions of diesel particulates have a well-established bimodal character that correspond to the particle nucleation and agglomeration mechanisms, with the corresponding particle types referred to as the nuclei mode and the accumulation mode respectively (see FIG. 1). As can be seen from FIG. 1, in the nuclei mode, diesel PM is composed of numerous small particles holding very little mass. Nearly all diesel particulates have sizes of significantly less than 1 μm, i.e. they comprise a mixture of fine, i.e. falling under the 1997 US law, ultrafine and nanoparticles.
Nuclei mode particles are believed to be composed mostly of volatile condensates (hydrocarbons, sulfuric acid, nitric acid etc.) and contain little solid material, such as ash and carbon. Accumulation mode particles are understood to comprise solids (carbon, metallic ash etc.) intermixed with condensates and adsorbed material (heavy hydrocarbons, sulfur species, nitrogen oxide derivatives etc.) Coarse mode particles are not believed to be generated in the diesel combustion process and may be formed through mechanisms such as deposition and subsequent re-entrainment of particulate material from the walls of an engine cylinder, exhaust system, or the particulate sampling system. The relationship between these modes is shown in FIG. 1.
The composition of nucleating particles may change with engine operating conditions, environmental condition (particularly temperature and humidity), dilution and sampling system conditions. Laboratory work and theory have shown that most of the nuclei mode formation and growth occur in the low dilution ratio range. In this range, gas to particle conversion of volatile particle precursors, like heavy hydrocarbons and sulfuric acid, leads to simultaneous nucleation and growth of the nuclei mode and adsorption onto existing particles in the accumulation mode. Laboratory tests (see e.g. SAE 980525 and SAE 2001-01-0201) have shown that nuclei mode formation increases strongly with decreasing air dilution temperature but there is conflicting evidence on whether humidity has an influence.
Generally, low temperature, low dilution ratios, high humidity and long residence times favour nanoparticles formation and growth. Studies have shown that nanoparticles consist mainly of volatile material like heavy hydrocarbons and sulfuric acid with evidence of solid fraction only at very high loads.
Contrastingly, engine-out size distributions of gasoline particulates in steady state operation show a unimodal distribution with a peak of about 60-80 nm (see e.g. FIG. 4 in SAE 1999-01-3530). By comparison with diesel size distribution, gasoline PM is predominantly ultrafine with negligible accumulation and coarse mode.
Particulate collection of diesel particulates in a diesel particulate filter is based on the principle of separating gas-borne particulates from the gas phase using a porous barrier. Diesel filters can be defined as deep-bed filters and/or surface-type filters. In deep-bed filters, the mean pore size of filter media is bigger than the mean diameter of collected particles. The particles are deposited on the media through a combination of depth filtration mechanisms, including diffusional deposition (Brownian motion), inertial deposition (impaction) and flow-line interception (Brownian motion or inertia).
In surface-type filters, the pore diameter of the filter media is less than the diameter of the PM, so PM is separated by sieving. Separation is done by a build-up of collected diesel PM itself, which build-up is commonly referred to as “filtration cake” and the process as “cake filtration”.
It is understood that diesel particulate filters, such as ceramic wallflow monoliths, may work through a combination of depth and surface filtration: a filtration cake develops at higher soot loads when the depth filtration capacity is saturated and a particulate layer starts covering the filtration surface. Depth filtration is characterized by somewhat lower filtration efficiency and lower pressure drop than the cake filtration.
WO 03/011437 discloses a gasoline engine having an exhaust system comprising means for trapping PM from the exhaust gas and a catalyst for catalysing the oxidation of the PM by carbon dioxide and/or water in the exhaust gas, which catalyst comprising a supported alkali metal. The means for trapping PM is suitable for trapping PM of particle range 10-100 nm, and can be a wallflow filter made from a ceramic material of appropriate pore size such as cordierite coated with the catalyst, a metal oxide foam supporting the catalyst, a wire mesh, a diesel wallflow filter designed for diesel applications, an electrophoretic trap or a thermophoretic trap (see e.g. GB-A-2350804).
WO 2008/136232 A1 discloses a honeycomb filter having a cell wall composed of a porous cell wall base material and, provided on its inflow side only or on its inflow and outflow sides, a surface layer and satisfying the following requirements (1) to (5) is used as a diesel particulate filter: (1) the peak pore diameter of the surface layer is identical with or smaller than the average pore diameter of the cell wall base material, and the porosity of the surface layer is larger than that of the cell wall base material; (2) with respect to the surface layer, the peak pore diameter is from 0.3 to less than 20 μm, and the porosity is from 60 to less than 95% (measured by mercury penetration method); (3) the thickness (L1) of the surface layer is from 0.5 to less than 30% of the thickness (L2) of the cell wall; (4) the mass of the surface layer per filtration area is from 0.01 to less than 6 mg/cm2; and (5) with respect to the cell wall base material, the average pore diameter is from 10 to less than 60 μm, and the porosity is from 40 to less than 65%. See also SAE paper no. 2009-01-0292.
Other techniques suggested in the art for separating gasoline PM from the gas phase include vortex recovery.
In the United States, no similar emission standards have been set. However, the State of California Air Resources Board (CARB) recently published a paper entitled “Preliminary Discussion Paper—Amendments to California's Low-Emission Vehicle [LEV] Regulations for Criteria Pollutants—LEV III” (release date 8 Feb. 2010) in which a new PM standard of between 2 and 4 mg PM/mile (1.25-2.50 mg PM/km (currently 10 mg PM/mile (6.25 mg PM/km))) is proposed, the paper commenting that: “Staff has received input from a number of manufacturers suggesting that a standard of 3 mg PM/mile (1.88 mg PM/km) can be met for gasoline direct injection engines without requiring the use of particulate filters.” Additionally, the paper states that since the PM mass and count emissions appear to be correlated: “Although a mandatory number standard is not being considered at this time, an optional PM number standard of about 1012 particles/mile [6.2511 particles/km] is being considered (which could be chosen by manufacturers instead of the PM mass standard)”. However, since neither the PM standard nor the PM number standard has been set by CARB yet, it is too soon to know whether particulate filtration will be necessary for the Californian or US vehicle market generally. It is nevertheless possible that certain vehicle manufacturers will choose filters in order to provide a margin of safety on any positive ignition engine design options selected to meet whatever standards are eventually set.
The new Euro 6 emission standard presents a number of challenging design problems for meeting gasoline emission standards. In particular, how to design a filter, or an exhaust system including a filter, for reducing the number of PM gasoline (positive ignition) emissions, yet at the same time meeting the emission standards for non-PM pollutants such as one or more of oxides of nitrogen (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC), all at an acceptable back pressure, e.g. as measured by maximum on-cycle backpressure on the EU drive cycle.
It is envisaged that a minimum of particle reduction for a three-way catalysed particulate filter to meet the Euro 6 PM number standard relative to an equivalent flowthrough catalyst is ≧50%. Additionally, while some backpressure increase for a three-way catalysed wallflow filter relative to an equivalent flowthrough catalyst is inevitable, in our experience peak backpressure over the MVEG-B drive cycle (average over three tests from “fresh”) for a majority of passenger vehicles should be limited to <200 mbar, such as <180 mbar, <150 mbar and preferably <120 mbar e.g. <100 mbar.
PM generated by positive ignition engines has a significantly higher proportion of ultrafine, with negligible accumulation and coarse mode compared with that produced by diesel (compression ignition) engines, and this presents challenges to removing it from positive ignition engine exhaust gas in order to prevent its emission to atmosphere. In particular, since a majority of PM derived from a positive ignition engine is relatively small compared with the size distribution for diesel PM, it is not practically possible to use a filter substrate that promotes positive ignition PM surface-type cake filtration because the relatively low mean pore size of the filter substrate that would be required would produce impractically high backpressure in the system.
Furthermore, generally it is not possible to use a conventional wallflow filter, designed for trapping diesel PM, for promoting surface-type filtration of PM from a positive ignition engine in order to meet relevant emission standards because there is generally less PM in positive ignition exhaust gas, so formation of a soot cake is less likely; and positive ignition exhaust gas temperatures are generally higher, which can lead to faster removal of PM by oxidation, thus preventing increased PM removal by cake filtration. Depth filtration of positive ignition PM in a conventional diesel wallflow filter is also difficult because the PM is significantly smaller than the pore size of the filter medium. Hence, in normal operation, an uncoated conventional diesel wallflow filter will have a lower filtration efficiency when used with a positive ignition engine than a compression ignition engine.
Another difficulty is combining filtration efficiency with a washcoat loading, e.g. of catalyst for meeting emission standards for non-PM pollutants, at acceptable backpressures. Diesel wallflow particulate filters in commercially available vehicles today have a mean pore size of about 13 μm. However, we have found that washcoating a filter of this type at a sufficient catalyst loading such as is described in US 2006/0133969 to achieve required gasoline (positive ignition) emission standards can cause unacceptable backpressure.
In order to reduce filter backpressure it is possible to reduce the length of the substrate. However, there is a finite level below which the backpressure increases as the filter length is reduced. Suitable filter lengths for filters according to embodiments of the present invention are from 2-12 inches long, preferably 3-6 inches long. Cross sections can be circular and in our development work we have used 4.66 and 5.66 inch diameter filters. However, cross-section can also be dictated by space on a vehicle into which the filter is required to fit. So for filters located in the so-called close coupled position, e.g. within 50 cm of the engine exhaust manifold where space is at a premium, elliptical or oval filter cross sections can be contemplated. As would be expected, backpressure also increases with washcoat loading and soot loading.
There have been a number of recent efforts to combine three-way catalysts with filters for meeting the Euro 6 emission standards.
US 2009/0193796 discloses a three-way conversion catalyst coated onto a particulate trap. The Examples disclose e.g. a soot filter having a catalytic material prepared using two coats: an inlet coat and an outlet coat. The mean pore size of the soot filter substrate used is not mentioned. The inlet coat contains alumina, an oxygen storage component (OSC) and rhodium all at a total loading of 0.17 g in−3; the outlet coat includes alumina, an OSC and palladium, all at a total loading of 0.42 g in−3. However, we believe that the three-way catalyst washcoat loading of <0.5 g in−3 provides insufficient three-way activity to meet the required emission standards alone, i.e. the claimed filter appears to be designed for inclusion in a system for location downstream of a three-way catalyst comprising a flowthrough substrate monolith.
WO 2009/043390 discloses a catalytically active particulate filter comprising a filter element and a catalytically active coating composed of two layers. The first layer is in contact with the in-flowing exhaust gas while the second layer is in contact with the out-flowing exhaust gas. Both layers contain aluminium oxide. The first layer contains palladium, the second layer contains an oxygen-storing mixed cerium/zirconium oxide in addition to rhodium. In Examples, a wallflow filter substrate of unspecified mean pore size is coated with a first layer at a loading of approximately 31 g/l and a second layer at a loading of approximately 30 g/l. That is, the washcoat loading is less than 1.00 g in−3. For a majority of vehicle applications, this coated filter is unlikely to be able to meet the required emission standards alone.
A difficulty in coating a filter with a catalyst composition is to balance a desired catalytic activity, which generally increases with washcoat loading, with the backpressure that is caused by the filter in use (increased washcoat loading generally increases backpressure) and filtration efficiency (backpressure can be reduced by adopting wider mean pore size and higher porosity substrates at the expense of filtration efficiency).